D'Arcy Wentworth Thompson
http://www.archive.org/stream/onaristotleasbio00thomrich#page/n3/mode/2up On Aristotle as a biologist. http://www.archive.org/stream/ongrowthform1917thom/ongrowthform1917thom_djvu.txt On Growth and Form. Thompson on natural selection The physiological speculations we need not discuss : but, to take a single example from morphology, we begin to understand the possibility, and to comprehend the probable meaning, of the * Cf. Loeb, Science, May 14, 191.5, f Cf. Baumann u. Roos, Vorkommen von lod im Thierkorper, Zeitschr. fur Physiol. Chem. xxi, xxii, 1895, 6. % Le Neo- Vitalisme, Rev. Scientifique, Mars 1911, p. 22 (of reprint). § La vie et la mort, p. 43, 1902. Ill] GEOWTH AND CATALYTIC ACTION 137 all but sudden appearance on the earth of such exaggerated and almost monstrous forms as those of the great secondary reptiles and the great tertiary mammals*. We begin to see that it is in order to account, not for the appearance, but for the disappearance of such forms as these that natural selection must be invoked. And we then, I think, draw near to the conclusion that what is true of these is universally true, and that the great function of natural selection is not to originate, but to remove : donee ad inter itum genus id natura redegitf. The world of things living, like the world of things inanimate, grows of itself, and pursues its ceaseless course of creative evolution. It has room, wide but not unbounded, for variety of living form and structure, as these tend towards their seemingly endless, but yet strictly limited, possibilities of permutation and degree : it has room for the great and for the small, room for the weak and for the strong; Environment and circumstance do not always make a prison, wherein perforce the organism must either live or die ; for the ways of life may be changed, and many a refuge found, before the sentence of unfitness is pronounced and the penalty of extermination paid. But there comes a time when "variation," in form, dimensions, or other qualities of the organism, goes farther than is compatible with all the means at hand of health and welfare for the individual and the stock ; when, under the active and creative stimulus of forces from within and from without, the active and creative energies of growth pass the bounds of physical and physiological equilibrium : and so reach the limits which, as again Lucretius tells us, natural law has set between what may and what may not be, "et quid quaeque queant per foedera naturai quid porro nequeant." Then, at last, we are entitled to use the customary metaphor, and to see in natural selection an inexorable force, whose function * Cf. Dendy, Evolutionary Biology, 1912, p. 408; Brit. Ass. Report (Portsmouth), 1911, p. 278. I Lucret. v, 877. "Lucretius nowhere seems to recognise the possibility of improvement or change of species by 'natural selection'; the animals remain as they were at the first, except that the weaker and more useless kinds have been crushed out. Hence he stands in marked contrast with modern evolutionists." Kelsey's note, ad loc. 138 THE RATE OF GROWTH is not to create but to destroy, — to weed, to prune, to cut down and to cast into the fire*. selection2 * Even after we have so narrowed the scope and sphere of natural selection, it is still hard to understand ; for the causes of extinction are often wellnigh as hard to comprehend as are those of the origin of species. If we assert (as has been lightly done) that Smilodon perished owing to its gigantic tusks, that Teleosaurus was handicapped by its exaggerated snout, or Stegosaurus weighed down by its intolerable load of armour, we may be reminded of other kindred forms to show that similar conditions did not necessarily lead to extermination, or that rapid extinction ensued apart from any such visible or apparent disadvantages. Cf. Lucas, F. A., On Momentum in Variation, Amer. Nat. xh, p. 46, 1907. t See Professor T. H. Morgan's Regeneration (316 pp.), 1901 for a full account and copious bibliography. The early experiments on regeneration, by Vallisneri, Reaumur, Bonnet, Trembley, Baster, and others, are epitomised by HaUer, Elem. Physiologiae, vm, p. 156 seq. J Journ. Experim. Zool. vii, p. ,397, 1909. selection3 Histologically, the case is illustrated by a well-known pheno- menon in embryology. In the segmenting ovum, there is a tendency for the cells to be budded off in linear series ; and so they often remain, in rows side by side, at least for a considerable time and during the course of several consecutive cell divisions. Such an arrangement constitutes what the embryologists call the "radial type" of segmentation*. But in what is described as the "spiral type" of segmentation, it is stated that, as soon as the first horizontal furrow has divided the cells into an upper and a lower layer, those of "the upper layer are shifted in respect to the lower layer, by means of a rotation about the vertical axisf." It is, of course, evident that the whole process is merely that which is familiar to physicists as "close packing." It is a very simple case of what Lord Kelvin used to call "a problem in tactics." It is a mere question of the rigidity of the system, of the freedom of movement on the part of its constituent cells, whether or at what stage this tendency to slip into the closest propinquity, or position of minimum potential, will be found to manifest itself. However the hexactinellid spicules be arranged (and this is * Se^, for instance, the figures of the segmenting egg of Synapta (after Selenka), in Korschelt and Heider's Vergleichende Entwicklutigsgescliichte (AUgem. Th., 3*<^ Lief.), p. 19, 1909. On the spiral type of segmentation as a secondary derivative, due to mechanical causes, of the "radial" type of segmentation, see E. B. Wilson, CeU-Uneage of Nereis, Journ. of Morphology, vi, p. 450, 1892. • •f Korschelt and Heider, p. 16. 454 ON CONCRETIONS, SPICULES, ETC. [ch. not at all easy to determine) in relation to the tissues and chambers of the sponge, it is at least clear that, whether they be separate or be fused together (as often happens) in a composite skeleton, they effect a symmetrical partitioning of space according to the cubical system, in contrast to that closer packing which is repre- sented and effected by the tetrahedral system*. This question of the origin and causation of the forms of sponge-spicules, with which we have noAv briefly dealt, is all the more important and all the more interesting because it has been discussed time and again, from points of view which are charac- teristic of very different schools of thought in biology. Haeckel found in the form of the sponge-spicule a typical illustration of his theory of "bio-crystalhsation"' ; he considered that these " biocrystals " represented "something midway — ein Mittelding — between an inorganic crystal and an organic secretion"; that there was a "compromise between the crystallising efforts of the calcium carbonate and the formative activity of the fused cells of the syncytium"; and that the semi-crystalline secretions of calcium carbonate " were utihsed by natural selection as ' spicules ' for building up a skeleton, and afterwards, by the interaction of adaptation and heredity, became modified in form and differen- tiated in a vast variety of ways in the struggle for existence f." What Haeckel precisely signified by these words is not clear to me. F. E. Schultze. perceiving that identical forms of spicule were developed whether the material were crystalline or non-crystalline, abandoned all theories based upon crystallisation ; he simply saw in the form and arrangement of the spicules something which was "best fitted" for its purpose, that is to say for the support and strengthening of the porous walls of the sponge, and found clear evidence of "utiUty" in the specific structure of these skeletal elements. selection4 IX OF THE SKELETON OF SPONGES 455 Sollas and Dreyer, as we have seen, introduced in various ways the conception of physical causation, — as indeed Haeckel himself had done in regard to one particular^ when he supposed the position of the spicules to be due to the constant passage of the water-currents. Though even here, by the way, if I under- stand Haeckel aright, he was thinking not merely of a direct or im- mediate physical causation , but of one manifesting itself through the agency of natural selection *. Sollas laid stress upon the " path of least resistance"' as determining the direction of growth; while Dreyer dealt in greater detail with the various tensions and pressures to which the growing spicule was exposed, amid the alveolar or vesicular structure which was represented alike by the chambers of the sponge, by the reticulum of constituent cells, or by the minute structure of the intracellular protoplasm. But neither of these writers, so far as I can discover, was inclined to doubt for a moment the received canon of biology, which sees in such structures as these the characteristics of true organic species, and the indications of an hereditary affinity by which blood-relationship and the succession of evolutionary descent throughout geologic time can be ultimately deduced. selection5 NOTE: Come back to tautology in this section Lastly, Minchin, in a well-known paper f, took sides with Schultze, and gave reasons for dissenting from such mechanical theories as those of Sollas and of Dreyer. For example, after pointing out that all protoplasm contains a number of "granules" or microsomes, contained in the alveolar framework and lodged at the nodes of the reticulum, he argued that these also ought to acquire a form such as the spicules possess, if it were the case that these latter owed their form to their very similar or identical position. "If vesicular tension cannot in any other instance cause the granules at the nodes to assume a tetraxon form, why should it do so for the sclerites ? " In all probability the answer to this question is not far to seek. If the force which the "mechanical" hypothesis has in view were simply that of mechanical pressure, * Op. cif. p. 483. " Die geordnete, oft so sehr regelmassige imd zierliche Zusam- mensetzung des Skeletsystems ist zum grossten Theile luimittelbares Product der Wasserstromung ; die characteristische Lagerung der Spicula ist von der constanten Richtung des Wasserstroms hervorgebracht ; zum kleinsten Theile ist sie die Folge von Anpassungen an untergeordnete aussere Existenzbedingungen." t Materials for a Monograph of the Ascones, Q. J. M. S. XL, pp. 469-587, 1898. 456 ON CONCRETIONS, SPICULES, ETC. [ch. as between solid bodies, then indeed we should expect that any substances whatsoever, lying between the impinging spheres, would tend (unless they were infinitely hard) to assume the quadriradiate or "tetraxon" form; but this conclusion does not follow at all, in so far as it is to surface-energy that we ascribe the phenomenon. Here the specific nature of the substances involved makes all the difference. We cannot argue from one substance to another ; adsorptive attraction shews its effect on one and not on another; and we have not the least reason to be surprised if we find that the little granules of protoplasmic material, which as they lie bathed in the more fluid protoplasm have (presumably, and as their shape indicates) a strong surface-tension of their own, behave towards the adjacent vesicles in a very different fashion to the incipient aggregations of calcareous or siliceous matter in a colloid medium. "The ontogeny of the spicules," says Professor Minchin, "points clearly to their regular form being a phylogetiefic adaptation, ivhicJi has become fixed and handed on by heredity, appearing in the ontogeny as a prophetic adaptation.^ And again, " The forms of the spicules are the result of adaptation to the requirements of the sponge as a whole, produced by the action of natural selection upon variation in every direction^ It would scarcely be possible to illustrate more briefly and more cogently than by these few w^ords (or the similar words of Haeckel quoted on p. 454), the fundamental difference between the Darwinian conception of the causation and determination of Form, and that which is characteristic of the physical sciences. If I have dealt comparatively briefly with the inorganic skeleton of sponges, in spite of the obvious importance of this part of our subject from the physical or mechanical point of view, it has been owing to several reasons. In the first place, though the general trend of the phenomena is clear, it must be at once admitted that many points are obscure, and could only be discussed at the cost of a long argument. In the second place, the physical theory is (as I have shewn) in manifest conflict with the accounts given by various embryologists of the development of the spicules, and of the current biological theories which their descriptions embody; it is beyond our scope to deal with such descriptions IX] OF THE RADIOLARIAN SKELETON 457 in detail. Lastly, we find ourselves able to illustrate the same physical principles with greater clearness and greater certitude in another group of animals, namely the Radiolaria. In our descrip- tion of the skeletons occurring within this group we shall by no means abandon the preliminary classification of microscopic skeletons which we have laid down ; but we shall have occasion to blend with it the consideration of certain other more or less correlated phenomena. selection6 '''Notes: Finally got a tautology in this section' ■f The whole phenomenon is described by biologists as a "surprising exhibition of constructive and selective activity," and is ascribed, in varying phraseology, to intelligence, skill, purpose, psychical actiArity, or "microscopic mentality": that is to say, to Galen's rex^i-Kr) 0i'>ais, or " artistic creativeness " (cf. Brock's Galen, 1916, p. xxix). Ci. CaTY>en.teT, Mental Physiology, 1874, p. 41; Norman, Architectural achievements of Little Masons, etc., Ann. Mag. Nat. Hist. (5), i, p. 284, 1878; Heron- AUen, Contributions... to the Study of the Forammifera, Phil. Trans. (B), ccvi, pp. 227-279, 1915 ; Theory and Phenomena of Purpose and Intelligence exhibited by the Protozoa, as illustrated by selection and behaviour in the Foraminifera, Journ. R. Microscop. Sac. pp. 547-557, 1915; ibid., pp. 137-140, 1916. Prof. J. A. Thomson (New Statesman, Oct. 23, 1915) describes a certam little foraminifer, whose proto- plasmic body is overlaid by a crust of sponge-spicules, as "a psycho-physical individuality whose experiments in self-expression include a masterly treatment of sponge-spicides, and illustrate that organic skill which came before the dawn of Art." Sir Ray Lankester finds it "not difficult to conceive of the existence of a mechanism in the protoplasm of the Protozoa which selects and rejects building-material, and determines the shapes of the structures built, comparable to that mechanism which is assumed to exist in the nervous system of insects and other animals which 'automatically' go through wonderfully elaborate series of complicated actions." And he agrees with "Darwin and others who have attributed the building up of these inherited mechanisms to the age-long action of Natural Selection, and the survival of those individuals possessing quaUties or 'tricks' of life-saving value," ''' J. R. Microsc. Soc. April, 1916, p. 136. selection7 '''Notes: In this section Thompson manages to get a glimpse of Popper unfalsifiability with ns In certain Ammonites the septal outline is further compUcated in another way. Superposed upon the usual sinuous outline, with its "lobes" and "saddles," we have here a minutely ramified, or arborescent outline, in which all the branches terminate in wavy. Fig. 307. Suture-line of a Tiiassic Ammonite (Pinacoccras). (From Zittel, after Hauer.) more or less circular arcs, — looking just Uke the 'landscape marble ' from the Bristol Rhaetic. We have no difficulty in recognising in this a surface-tension phenomenon. The figures are precisely such as we can imitate (for instance) by pouring a XI] CONCLUSION 585 few drops of milk upon a greasy plate, or of oil upon an alkaline solution. We have very far from exhausted, we have perhaps little more than begun, the study of the logarithmic spiral and the associated curves which find exemplification in the multitudinous diversities of molluscan shells. But, with a closing word or two, we must now bring this chapter to an end. In the spiral shell we have a problem, or a phenomenon, of growth, immensely simplified by the fact that each successive increment is irrevocably fixed in regard to magnitude and position, instead of remaining in a state of flux and sharing in the further changes which the organism undergoes. In such a structure, then, we have certain primary phenomena of growth manifested in their original simplicity, undisturbed by secondary and conflicting phenomena. What actually grotvs is merely the lip of an orifice, w^here there is produced a ring of solid material, whose form we have treated of under the name of the generating curve ; and this generating curve grows in magnitude without alteration of its form. Besides its increase in areal magnitude, the growing curve has certain strictly limited degrees of freedom, which define its motions in space : that is to say, it has a vector motion at right angles to the axis of the shell ; and it has a sliding motion along that axis. And, though we may know nothing whatsoever about the actual velocities of any of these motions, we do know that they are so correlated together that their relative velocities remain constant, and accordingly the form and symmetry of the whole system remain in general unchanged. But there is a vast range of possibilities in regard to every one of these factors : the generating curve may be of various forms, and even when of simple form, such as an ellipse, its axes may be set at various angles to the system ; the plane also in which it lies may vary, almost indefinitely, in its angle relatively to that of any plane of reference in the system ; and in the several velocities of growth, of rotation and of translation, and therefore in the ratios between all these, we have again a vast range of possibihties. We have then a certain definite type, or group of forms, mathematically isomorphous, but presenting infinite diver- sities of outward appearance : which diversities, as Swammerdam 586 THE LOGARITHMIC SPIRAL [ch. xi said, ex sola nascuntur diversitate gyrationum ; and which accord- ingly are seen to have their origin in differences of rate, or of magnitude, and so to be, essentially, neither more nor less than differences of degree. In nature, we find these forms presenting themselves with but little relation to the character of the creature by which they are produced. Spiral forms of certain particular kinds are common to Gastropods and to Cephalopods, and to diverse families of each; while outside the class of molluscs altogether, among the Foraminifera and among the worms (as in Spirorbis, Spirographs, and in the Dentalium-like shell of Ditrupa), we again meet with /similar and corresponding forms. Again, we find the same forms, or forms which (save for external ornament) are mathematically identical, repeating themselves in all periods of the world's geological history ; and, irrespective of climate or local conditions, we see them mixed up, one with another, in the depths and on the shores of every sea. It is hard indeed (to my mind) to see where Natural Selection necessarily enters in, or to admit that it has had any share whatsoever in the production of these varied conformations. Unless indeed we use the term Natural Selection in a sense so wide as to deprive it of any purely biological significance; and so recognise as a sort of natural selection whatsoever nexus of causes sufiices to differ- entiate between the likely and the unlikely, the scarce and the frequent, the easy and the hard : and leads accordingly, under the pecuUar conditions, limitations and restraints which we call "ordinary circumstances," one type of crystal, one form of cloud, one chemical compound, to be of frequent occurrence and another to be rare. selection8 notes: Fibonacci sequences in leaves and ns is to vary, to make blind shots at constructions, or to 'mutate as it is now termed ; and the most suitable of these constructions will in the long run be isolated by the action of Natural Selection.' Finally, and this is the most concrete objection of all, the supposed isolation of the leaves, or their most complete "distribution to the action of the surrounding atmosphere" is manifestly very little affected by any conditions which are confined to the angle of azimuth. If we could imagine a case in which all the leaves of the stem, or all the scales of a fir-cone, were crushed down to one and the same level, into a simple ring or whorl of leaves, then indeed they would have their most equable distribution under the condition of the "ideal angle," that is to say of the "golden mean." But if it be (so to speak) Nature's object to set them further apart than they actually are, to give them freer exposure to the air than they actually have, then it is surely manifest that the simple way to do so is to elongate the axis, and to set the leaves further apart, lengthways on the stem. This has at once a far more potent effect than any nice manipulation of the "angle of divergence." For it is obvious that in i^(^ . sin 6) we have a greater range of variation by altering 6 than by altering 4>. We come then, without more ado, to the conclusion that the "Fibon- acci series," and its supposed usefulness, and the hypothesis of its introduction into plant-structure through natural selection, are all matters which deserve no place in the plain study ol botanical phenomena. As Sachs shrewdly recognised years ago. all such speculations as these hark back to a school of mystical idealism. selection9 In more recent times, other theories, based upon the principles of Natural Selection, have been current and very generally accepted, to account for these diversities of form. The pointed, conical egg of the guillemot is generally supposed to be an adaptation, * Cf. Lapierre, in Buffon's Histoire Naturelle, ed. Sonnini, 1800. t Eier 4er Vogel Deutschlands, 1818-28 (cit. des Murs, p. 36). t Traite d'Oologie, 1860, § Lafresnaye, F. de, Comparaison des cBufs des Oiseaux avec leurs squelettes, comme seul moven de reconnaitre la cause de leurs differentes formes, Bev. ZooL, 1845, pp. lSO-187, 239-244. II a. Des Murs, p. 67: "EUe devait encore penser au moment ou ce germe aurait besoin de I'espace necessaire a son accroissement, a, ce moment ou...il devra remplir exactement I'intervalle circonscrit par sa fragile prison, etc." ^ Thienemann, F. A. L., Syst. Darstellung der Fortpflanzung der Vogel Europas, Leipzig, 1825-38. 654 ON THE SHAPES OF EGGS advantageous to the species in the circumstances under which the egg is laid ; the pointed egg is less apt than a spherical one to roll off the narrow ledge of rock on which this bird is said to lay its sohtary egg, and the more pointed the egg, so much the fitter and UkeUer is it to survive. The fact that the plover or the sandpiper, breeding in very different situations, lay eggs that are also conical, ehcits another explanation, to the effect that here the conical form permits the many large eggs to be packed closely under the mother bird * . Whatever truth there be in these apparent adaptations to existing circumstances, it is only by a very hasty logic that we can accept them as a vera causa, or adequate explanation of the facts ; and it is obvious that, in the bird's egg, we have an admirable case for the direct investigation of the mechanical or physical significance of its formf. selection10 To buttress the theory of natural selection the same instances of "adaptation" (and many more) are used, which in an earher but not distant age testified to the wisdom of the Creator and revealed to simple piety the high purpose of God. In the words of a certain learned theologian J, "The free use of final causes to explain what seems obscure was temptingly easy.... Hence the finahst was often the man who made a liberal use of the ignava ratio, or lazy argument: when you failed to explain a thing by the ordinary process of causahty, you could "explain" it by reference to some purpose of nature or of its Creator. This method lent itself with dangerous facihty to the well-meant endeavours of the older theologians to expound and emphasise the beneficence of the divine purpose." Mutatis mutandis, the passage carries its plain message to the uaturahst. The fate of such arguments or illustrations is always the same. They attract and captivate for awhile; they go to the building of a creed, which contemporary orthodoxy defends under its severest penalties : but the time comes when they lose their fascination, they somehow cease to satisfy and to convince, their foundations are discovered to be insecure, and in the end no man troubles to controvert them. But of a very different order from all such "adaptations" as these, are those very perfect adaptations of form which, for instance, fit a fish for swimming or a bird for flight. Here we are * Nature, l, p. 572; li, pp. 33, 57, 533, 1894-95. f They are "wonderfully fitted for 'vanishment' against the flushed, rich- coloured skies of early morning and evening.... their chief feeding-times"; and "look like a real sunset or dawn, repeated on the opposite side of the heavens, — either east or west as the case may be": Thayer, Concealing -coloration in the Animal Kingdom, New York, 1909, pp. 154-155. This hypothesis, like the rest, is not free from difficulty. Twilight is apt to be short in the homes of the flamingo : and moreover. Mr Abel Chapman, who watched them on the Guadalquivir, tells us that they feed by day. % Principal Galloway, Philosophy of Religion, p. 344, 1914. XVI THE PROBLEM OF ADAPTATION 673 far above the region of mere hypothesis, for we have to deal with questions of mechanical efficiency where statical and dynamical considerations can be applied and established in detail. The naval architect learns a great part of his lesson from the investi- gation of the stream-lines of a fish ; and the mathematical study of the stream-lines of a bird, and of the principles underlying the areas and curvatures of its wings and tail, has helped to lay the very foundations of the modern science of aeronautics. When, after attempting to comprehend the exquisite adaptation of the swallow or the albatross to the navigation of the air, we try to pass beyond the empirical study and contemplation of such perfection of mechanical fitness, and to ask how such fitness came to be, then indeed we may be excused if we stand wrapt in wonder- ment, and if our minds be occupied and even satisfied with the conception of a final cause. And yet all the while, with no loss of wonderment nor lack of reverence, do we find ourselves con- strained to believe that somehow or other, in dynamical principles and natural law, there lie hidden the steps and stages of physical causation by which the material structure was so shapen to its ends*. But the problems associated with these phenomena are difficult at every stage, even long before we approach to the unsolved secrets of causation ; and for my part I readily confess that I lack the requisite knowledge for even an elementary discussion of the form of a fish or of a bird. But in the form of a bone we have a problem of the same kind and order, so far simplified and particularised that we may to some extent deal with it, and may possibly even find, in our partial comprehension of it, a partial clue to the principles of causation underlying this whole class of problems. Irreducible Complexity notes: Highlight Thompson seemingly first glimpse of Behe's IC or Irreducible Functionality concept NOTES: Thompson called it composite integrity In various ways our structural problem is beset by "limiting conditions." Not only must rigidity be associated with flexibility, but also stability must be ensured in various positions and attitudes ; and the primary function of support or weight-carrying must be combined with the provision of points (Vappui for the muscles concerned in locomotion. We cannot hope to arrive at a numerical or quantitative solution of this complicate problem, but we have found it possible to trace it out in part towards a qualitative solution. And speaking broadly we may certainly say that in each case the problem has been solved by nature herself, very much as she solves the difficult problems of minimal areas in a system of soap-bubbles; so that each animal is fitted with a backbone adapted to his own individual needs, or (in other words) corresponding exactly to the mean resultant of the stresses to which as a mechanical system it is exposed. Throughout this short discussion of the principles of con- struction, limited to one part of the skeleton, we see the same general principles at work which we recognise in the plan and construction of an individual bone. That is to say, we see a tendency for material to be laid down just in the lines of stress, and so as to evade thereby the distortions and disruptions due to shear. In these phenomena there lies a definite law of growth, 712 ON FORM AND MECHANICAL EFFICIENCY whatever its ultimate expression or explanation may come to be. Let us not press either argument or hypothesis too far: but be content to see that skeletal form, as brought about by growth, is to a very large extent determined by mechanical considerations, and tends to manifest itself as a diagram, or reflected image, of mechanical stress. If we fail, owing to the immense complexity of the case, to unravel all the mathematical principles involved in the construction of the skeleton, we yet gain something, and not a little, by applying this method to the familiar objects of our anatomical study: obvia conspicimus, nubem pellente mathesi*. Before we leave this subject of mechanical adaptation, let us dwell once more for a moment upon the considerations which arise from our conception of a field of force, or field of stress, in which tension and compression (for instance) are inevitably combined, and are met by the materials naturally fitted to resist them. It has been remarked over and over again how harmoni- ously the whole organism hangs together, and how throughout its fabric one part is related and fitted to another in strictly functional correlation. But this conception, though never denied, is sometimes apt to be forgotten in the course of that process of more and more minute analysis by which, for simplicity's sake, we seek to unravel the intricacies of a complex organism. We tend, as we analyse a thing into its parts or into its properties, to magnify these, to exaggerate their apparent independence, and to hide from ourselves (at least for a time) the essential integrity and individuality of the composite whole. We divide the body into its organs, the skeleton into its bones, as in very much the same fashion we make a subjective analysis of the mind, according to the teachings of psychology, into component factors: but we know very well that judgment and knowledge, courage or gentleness, love or fear, have no separate existence, but are somehow mere manifestations, or imaginary co-efficients, of a most complex integral. And likewise, as biologists, we may go so far as to say that even the bones themselves are only in a limited and even a deceptive sense, separate and individual things. The skeleton begins as a continuum, and a continuum it remains all life long. The things that link bone with bone, * The motto was Macquorn Rankine's. XVI ON THE SKELETON AS A WHOLE 713 cartilage, ligaments, membranes, are fashioned out of the same primordial tissue, and come into being -pari jmssu, with the bones themselves. The entire fabric has its soft parts and its hard, its rigid and its flexible parts ; but until we disrupt and dismember its bony, gristly and fibrous parts, one from another, it exists simply as a "skeleton," as one integral and individual whole. A bridge was once upon a time a loose heap of pillars and rods and rivets of steel. But the identity of these is lost, just as if they were fused into a solid mass, when once the bridge is built; their separate functions are only to be recognised and analysed in so far as we can analyse the stresses, the tensions and the pressures, which affect this part of the structure or that; and these forces are not themselves separate entities, but are the resultants of an analysis of the whole field of force. Moreover when the bridge is broken it is no* longer a bridge, and all its strength is gone. So is it precisely with the skeleton. In it is reflected a field of force : and keeping pace, as it were, in action and interaction with this field of force, the whole skeleton and every part thereof, down to the minute intrinsic structure of the bones themselves, is related in form and in position to the lines of force, to the resistances it has to encounter; for by one of the mysteries of biology, resistance begets resistance, and where pressure falls there growth springs up in strength to meet it. And, pursuing the same train of thought, we see that all this is true not of the skeleton alone but of the whole fabric of the body. Muscle and bone, for instance, are inseparably associated and connected ; they are moulded one with another ; they come into being together, and act and react together*. We may study them apart, but it is as a concession to our weakness and to the narrow outlook of our minds. We see, dimly perhaps, but yet with all the assurance of conviction, that between muscle and bone there can be no change in the one but it is correlated with changes in the other; that through and through they are linked in indissoluble association ; that they are only separate entities * John Hunter was seldom wrong ; but I cannot believe that he was right when he said (Scientific Works, ed. Owen, i, p. 371), "The bones, in a mechanical view, appear to be the first that are to be considered. We can study their shape, connections, number, uses, etc., without considering any other part of the body.^' 714 ON FORM AND MECHANICAL EFFICIENCY [ch. in this limited and subordinate sense, that they are parts of a whole which, when it loses its composite integrity, ceases to exist. The biologist, as well as the philosopher, learns to recognise that the whole is not merely the sum of its parts. It is this, and much more than this. For it is not a bundle of parts but an organisation of parts, of parts in their mutual arrangement, fitting one with another, in what Aristotle calls "a single and indivisible principle of unity" ; and this is no merely metaphysical conception, but is in biology the fundamental truth which lies at the basis of Geoffroy's (or Goethe's) law of "compensation," or "balancement of growth." Nevertheless Darwin found no difficulty in believing that "natural selection will tend in the long run to reduce any part of the organisation, as soon as, through changed habits, it becomes superfluous : without by any means causing some other part to be largely developed in a corresponding degree. And conversely, that natural selection may perfectly well succeed in largely deve- loping an organ without requiring as a necessary compensation the reduction of some adjoining part*." This view has been developed into a doctrine of the "independence of single char- acters" (not to be confused with the germinal "unit characters" of Mendelism), especially by the palaeontologists. Thus Osborn asserts a "principle of hereditary correlation," combined with a " principle of hereditary separability whereby the body is a colony, a mosaic, of single individual and separable charactersf-" I cannot think that there is more than a small element of truth in this doctrine. As Kant said, "die Ursache der Art der Existenz bei jedem Theile eines lebenden Korpers ist im Ganzen enthalten..'^ And, according to the trend or aspect of our thought, we may look upon the co-ordinated parts, now as related and fitted to the end or function of the whole, and now as related to or resulting from the physical causes inherent in the entire system of forces to which the whole has been exposed, and under whose influence it has come into being J. * Origin of Species, 6th ed. p. 118. t Atner. Naturalist, April, 1915, p. 198, etc. Cf. infra, p. 727. J Driesch sees in "Entelechy" that something which differentiates the whole XVI] THE PROBLEM OF PHYLOGENY 715 It would seem to me that the mechanical principles and phenomena which we have dealt with in this chapter are of no small importance to the morphologist, all the more when he is inclined to direct his study of the skeleton exclusively to the problem of phylogeny; and especially when, according to the methods of modern comparative morphology, he is apt to take the skeleton to pieces, and to draw from the comparison of a series of scapulae, humeri, or individual vertebrae, conclusions as to the descent and relationship of the animals to which they belong. It would, I dare say, be a gross exaggeration to see in every bone nothing more than a resultant of immediate and direct physical or mechanical conditions ; for to do so would be t® deny the existence, in this connection, of a principle of heredity. And though I have tried throughout this book to lay emphasis on the direct action of causes other than heredity, in short to circum- scribe the employment of the latter as a working hypothesis in morphology, there can still be no question whatsoever but that heredity is a vastly important as well as a mysterious thing; it is one of the great factors in biology, however we may attempt to figure to ourselves, or howsoever we may fail even to imagine, its underlying physical explanation. But I maintain that it is no less an exaggeration if we tend to neglect these direct physical and mechanical modes of causation altogether, and to see in the characters of a bone merely the results of variation and of heredity, and to trust, in consequence, to those characters as a sure and certain and unquestioned guide to affinity and phylogeny. Comparative anatomy has its physiological side, which filled men's minds in John Hunter's day, and in Owen's day ; it has its from the sum of its parts in the case of the organism: "The organism, we know, is a system the single constituents of which are inorganic in themselves ; only the whole constituted by them in their typical order or arrangement owes its specificity to 'Entelechy'" {Gifford LerAures, p. 2"?9, 1908): and I think it could be shewn that many other philosophers have said precisely the same thing. So far as the argument goes, I fail to see how this Entelechy is shewn to be peculiarly or specifically related to the living organism. The conception that the whole is ahvays somethini^ very different from its parts is a very ancient doctrine. The reader will perhaps remember how, in another vein, the theme is treated by Martinus Seriblerus: "In every Jack there is a meat-roasting Quality, which neither resides in the fly, nor in the weight, nor in any particular wheel of the Jack, but is the result of the whole composition; etc., etc." 716 ON FOKM AND MECHANICAL EFFICIENCY [ch. classificatory and phylogenetic aspect, which has all but filled men's minds during the last couple of generations; and we can lose sight of neither aspect without risk of error and misconception. It is certain that the question of phylogeny, always difficult, becomes especially so in cases where a great change of physical or mechanical conditions has come about, and where accordingly the physical and physiological factors in connection with change of form are bound to be large. To discuss these questions at length would be to enter on a discussion of Lamarck's philosophy of biology, and of many other things besides. But let us take one single illustration. The affinities of the whales constitute, as will be readily admitted, a very hard problem in phylogenetic classification. We know now that the extinct Zeuglodons are related to the old Creodont carnivores, and thereby (though distantly) to the seals ; and it is supposed, but it is by no means so certain, that in turn they are to be considered as representing, or as allied to, the ancestors of the modern toothed whales*. The proof of any such a contention becomes, to my mind, extraordinarily difiicult and complicated ; and the arguments commonly used in such cases may be said (in Bacon's phrase) to allure, rather than to extort assent. Though the Zeuglodonts were aquatic animals, we do not know, and we have no right to suppose or to assume, that they swam after the fashion of a whale (any more than the seal does), that they dived like a whale, and leaped like a whale. But the fact that the whale does these things, and the way in which he does them, is reflected in many parts of his skeleton — perhaps more or less in all: so much so that the lines of stress which these actions impose are the very plan and working-diagram of great part of his structure. That the Zeuglodon has a scapula like that of a whale is to my mind no necessary argument that he is akin by blood-relationship to a whale : that his dorsal vertebrae are very different from a whale's is no conclusive argument that * "There can be no doubt that Fraas is correct in regarding this type (Procetus) as an annectant form between the Zeuglodonts and the Creodonta, but, although the origin of the Zeuglodonts is thus made clear, it still seems to be by no means so certain as that author believes, that they may not themselves be the ancestral forms of the Odontoceti"; Andrews, Tertiary Vertebrata of the Fayum, 1906, p. 235. selection12 'Notes: ' If, in the evolution of a fish, for instance, it be the case that its several and constituent parts — head, body, and tail, or this fin and that fin — represent so many independent variants, then our co-ordinate system will at once become too complex to be intelligible; we shall be making not one comparison but several * Historia Animalium i, 1. XVII] THE COMPARISON OF RELATED FORMS 727 separate comparisons, and our general method will be found inapplicable. Now precisely this independent variability of parts and organs — here, there, and everywhere within the organism — would appear to be implicit in our ordinary accepted notions regarding variation; and, unless I am greatly mistaken, it is precisely on such a conception of the easy, frequent, and normal independent variability of parts that our conception of the process of natural selection is fundamentally based. For the morphologist, when comparing one organism with another, describes the differences between them point by point, and "character" by "character*." If he is from time to time constrained to admit the existence of "correlation" between characters (as a hundred years ago Cuvier first showed the way), yet all the while he recognises this fact of correlation somewhat vaguely, as a pheno- menon due to causes which, except in rare instances, he can hardly hope to trace ; 'and he falls readily into the habit of thinking and talking of evolution as though it had proceeded on the lines of his own descriptions, point by point, and character by characterf. ' selection13 Natural selection 4, 58, 137, 456, 586, 609, 651, 653 Category:TauTology